1. Field of the Invention
The invention relates to a process for preparing tert-butanol by addition of water onto isobutene in the presence of an acid ion exchanger in a reactor cascade in which mixtures having different tert-butanol and water contents are fed alternately into at least one reactor.
2. Discussion of the Background
tert-Butanol (TBA) is an important product which is prepared on a large industrial scale and is used as solvent and as intermediate for the preparation of methyl methacrylate. It is a precursor for the preparation of peroxides such as peroxy ketals, peresters or dialkyl peroxides having at least one tertiary butyl group. These compounds are used as oxidants and as initiators for free-radical reactions, for example olefin polymerization or crosslinking of plastics. tert-Butanol serves as intermediate for the recovery of pure isobutene from isobutene mixtures. Furthermore, it is a reagent for the introduction of tertiary butyl groups. Its alkali metal salts are strong bases which are used in many syntheses.
Tertiary butanol can be obtained by acid-catalyzed addition of water onto isobutene. Industrial isobutene mixtures frequently further comprise other olefins such as 2-butenes. If these starting materials are used, industrial processes employ conditions under which virtually only the isobutene and not the other olefins are hydrated and secondary reactions such as homo-oligomerization or heterooligomerization of the olefins are virtually completely suppressed. Such processes are usually carried out in the liquid phase and can be divided into two groups: a) processes in which the reaction is carried out in a water-containing catalyst solution and b) heterogeneous catalytic processes in which solid catalysts which are insoluble in the reaction phase are used.
The hydration of isobutene to tert-butanol with the aid of solid acid catalysts which are soluble neither in the starting materials nor in the products has the advantage that the reaction mixture is free of acid and can be worked up to obtain tert-butanol without losses caused by redissociation or by other secondary reactions. The reaction proceeds on the surface of the catalyst. For a reaction to take place, both reactants have to be present at the same time at the active site of the catalyst. This is made difficult by the fact that water and isobutene or an isobutene-containing hydrocarbon mixture are not miscible with one another. To obtain acceptable conversions, solvents which make a homogeneous mixture of water and isobutene starting mixture possible are used.
In DE 30 31 702 A1, methanol is described for this purpose as solvent both for water and for isobutene or an isobutene-containing hydrocarbon mixture. As products, tert-butanol and methyl tert-butyl ether are obtained side by side.
In EP 0 010 993 A1, aliphatic carboxylic acids having from 1 to 6 carbon atoms are used as solvents for both starting materials. In this case, the tertiary butyl esters of these acids are formed as by-products. These have to be hydrolyzed to tert-butanol and carboxylic acids.
Sulfolanes are used in DE 30 31 702 A1 and polyhydric alcohols of the neo type, for example neopentyl glycol, are used in U.S. Pat. No. 4,327,231. These solvents have to be separated off from the tert-butanol. In addition, there is a risk that the solvent used will decompose in a long-term operation.
To avoid these disadvantages, the target product, viz. TBA, is used as solubilizer in some processes. Such processes are described, for example, in WO 99/33775, DE 30 25 262 or DE 102 59 413.9. Here, a mixture of a hydrocarbon fraction in which isobutene is present, TBA and water is reacted over acid catalysts arranged in a fixed bed in a reactor cascade. The first reactor is usually operated in recirculation mode and the others are operated in a single pass. Water for the reaction can be added before each further reactor. The output from the last reactor is separated by distillation into a hydrocarbon mixture comprising the unreacted isobutene and crude TBA. Part of the crude TBA is recirculated to the first reactor. The other part can be utilized as such or worked up to give TBA and/or a TBA/water azeotrope.
In these processes, the TBA content increases when the isobutene content decreases from reactor to reactor due to progress of the reaction. The composition of the reaction mixture approaches the thermodynamic equilibrium between water, isobutene and TBA at a decreasing rate, so that complete conversion cannot be achieved. Conversions of about 93% can be achieved at long reaction times. However, if a higher space-time yield is desired, it is advantageous to limit the conversion. For example, it can be advantageous to limit the conversion when using industrial isobutene streams, for example raffinate I, to from 80 to 85%.
Patent application DE 103 38 581 describes the preparation of tert-butanol by reactingisobutene-containing hydrocarbon mixtures with water over solid acid catalysts in a plurality of reactors, with part of the TBA present in the reaction mixture being separated off upstream of the last reactor so as to increase the conversion. Despite this improvement, the total isobutene conversion at an approximately constant space-time yield increases by only 5–10% to about 90%.
Patent application DE 102 60 991 describes a process for preparing TBA in which a relatively complicated reactive distillation is used and in which high boilers are also formed from isobutene at virtually quantitative isobutene conversion.